Certain types of chemical, in particular biochemical, assays involve immobilising on a test substrate a probe species capable of binding selectively to a target species. A fluid sample, containing or suspected to contain the target species, is brought into contact with the test substrate; target species present in the sample will then bind to the immobilised probe. After washing the substrate to remove unbound species, the presence of the target-probe pair can be detected in several known ways, including via chemical “labels” (for instance, labels capable of chemiluminescence or fluorescence) attached to the target species.
This principle is used in a large number of biochemical assays, for instance to detect the presence of target nucleotide sequences or proteins. It involves, however, an often complex sequence of procedures. A suitably selective probe for the target species must firstly be identified, usually by means of some form of screening, and immobilised on a test substrate. A sample fluid must then be maintained in contact with the substrate for a sufficient period of time, and under suitable conditions, for target-probe binding to take place (and to take place to a detectable degree). During this period, the temperature of the sample often needs to be cycled between quite precise ranges and over specific time periods, to enable binding to occur. The test substrate must then be washed, usually with increasing levels of stringency to remove not only unbound species but also those which are bound with an unacceptably low degree of specificity. Finally, the washed substrate must be analysed to detect the presence and/or amount of target-probe pairs.
These procedures can to an extent be automated, but often still involve significant manual intervention, for instance to control the introduction of samples and reagents at appropriate times and locations. Moreover, apparatus for carrying out the procedures can be both complex and costly, involving large numbers of separate fluid control devices (valves and pumps) in order to introduce what is often a large number of necessary sample and/or reagent fluids.
Since it may be desirable to assay a large number of samples at a time, and/or to test a sample against a large number of probe species, there is a constant need to enhance the efficiency of such assays, to reduce the complexity of the apparatus in which they are carried out, to minimise the amount of manual intervention needed, to maximise throughput and/or to increase accuracy and consistency in the results. Furthermore, since the samples being assayed are often scarce (for instance, DNA-containing samples), and typically need to be screened for more than one target species, it is always desirable to minimise the amount of sample needed for an assay, typically by increasing detection sensitivity.
It is already known to carry out a chemical assay by spreading a thin layer of a liquid sample over a flat test substrate, such as a glass microscope slide, on which an “array” of several, often hundreds or more, probe species has been immobilised. This allows the sample to be screened simultaneously for a corresponding number of target species. Such arrays have, for instance, been disclosed recently for the detection of proteins in a biological sample; the test substrate may be referred to as a “protein array” or “protein biochip” [de Wildt, R M T et al, Nat Biotechnol, 18 (9), 989-94 (September 2000); Mendoza, G, BioTechniques, 27 (4), 781-788 (1999); Bussow, K et al, Genomic 65, 1-8 (2000)]. It would be desirable to be able to use such substrates in an at least partly automated assay process, and preferably to be able to process a plurality of substrates simultaneously.